The process and the device of the invention relate to the continuous introduction, at a stable and continuous flow rate, of zirconium tetrachloride, hafnium tetrachloride and mixtures thereof into a column for continuous extractive distillation under pressure of such chlorides.
The principle of feeding a column for the extractive distillation of zirconium and hafnium tetrachlorides in "hafnium-containing crude zirconium tetrachloride" vapour resulting from sublimation is known from the patent FR-C2250707. However, experience has shown that it is particularly difficult to introduce such vapour continuously under a "pressure" of between 0.0005 and 0.15 MPa and at a stable, sufficiently well known flow rate.
It should be noted that, in this text, and as conventional for internal pressures, the term "pressure" denotes the excess pressure relative to atmospheric pressure.
In fact, it is known that such chlorides can be introduced into a container by means of a screw which is possibly vibrated. If this container is under pressure, the screw lifts gas or sublimed vapour originating from the powder introduced. If the gaseous phase does not condense in the screw, its ascent through the screw obstructs or prevents the flow of powder, depending on the value of the pressure in this gaseous phase. If the gaseous phase condenses, a paste which tends to obstruct the screw is formed in the screw. In the case of zirconium tetrachloride, ZrCl.sub.4, which sublimes at about 335.degree. C. at atmospheric pressure, its conveyance by a screw, which may or may not be vibrated, into a sublimation apparatus at 335.degree. C. causes ascent of sublimated ZrCl.sub.4 vapour, which condenses in the screw and blocks it.
To overcome such disadvantages, various modifications of this introduction process have been tested by attempting either to balance the pressures on the two sides of the introduction screw, for example by closing the hopper for supplying the screw with powder and by injecting a small flow of neutral gas on this side of the screw, or to counteract the ascent of compressed gaseous phase in the screw, for example by injecting a neutral gas such as nitrogen into the container under pressure at the end of the screw so as to produce a cushion of inert gas. This second modification, which is appropriate ,for a continuous supply, does not completely prevent ascent of gaseous phase and condensation in the case of ZrCl.sub.4. It is thus necessary to prevent it more completely by additional variations, for example by adding a means for heating the nozzle at the screw outlet or, on the other hand, by cooling this nozzle and scraping the crust of condensed ZrCl.sub.4 as it begins to form, but these measures pose awkward mechanical problems.
However, these modifications do not provide a valid solution if the container is a column for continuous extractive distillation under pressure. In fact,
the injection of inert gas interrupts operation of the column
the flow of chloride vapours cannot be controlled with precision greater than a relative 1%
air can be introduced into the container with the chloride grains, and the oxygen contained is therefore a source of corrosion of the column material.
However, the variations and the imprecision in the control of the flow of vaporised chlorides introduced into the column, in particular, constitute the major drawback of the methods of the prior art.
This is why the applicants have sought a means of introducing, in particular, hafnium-containing zirconium tetrachloride into a column for extractive distillation of hafnium tetrachloride which is more accurate and more reliable, in which neutral gas is not added, even in a small quantity, and which is capable of operating under a pressure difference between a low value, for example 0.0005 MPa, and at least 0.15 MPa, this pressure difference being the difference existing between the sublimation apparatus and the column on the one hand and the atmosphere on the other hand.
Since the zirconium chlorides, hafnium chloride and mixtures thereof are soluble in certain molten salts, as described in French patent FR-C-2250707, the applicants have had the idea of using such solvents in the process according to the invention and, in particular, the complex potassium chloroaluminate solvent, KAlCl.sub.4 or (KCl,AlCl.sub.3).
In fact, tests have shown that this complex solvent is barely volatile. Its KCl vapour pressure is almost zero, its AlCl.sub.3 vapour pressure is low and these zero or low vapour pressure are not troublesome in the process of extractive distillation in the patent FR-C-2250707. At 335.degree. C., the sublimation temperature of ZrCl.sub.4 at atmospheric pressure, this solvent dissolves a quantity of ZrCl.sub.4 greater than 40 g per 100 g of solvent, and when the solution obtained is reheated to about 500.degree. C., more than 90% of ZrCl.sub.4 dissolved therein escapes in the form of vapour. Such a solution containing 40 g of ZrCl.sub.4 in 100 g of potassium chloroaluminate is homogeneous and is therefore pumpable between about 300.degree. and 350.degree. C. Its flow rate is thus continuously measurable and controllable.
However, according to the patent FR-C-2250707, other complex solvents can be used as, like KAlCl.sub.4, they have significant differences in solubility of zirconium and hafnium tetrachlorides with temperature and pressure and allow the dissolved ZrCl.sub.4 and HfCl.sub.4 to be recovered. These are: NaAlCl.sub.4, KFeCl.sub.4 and NaFeCl.sub.4. However, those which contain potassium have unexpectedly been found to be more favourable than those containing sodium with regard to the total solubility of the Zr and Hf chlorides, their relative volatility and their stability because they lead to good reliability of operating of the distillation installation and to constancy in the quality of the hafnium-containing zirconium chloride content.